1. Field of the Invention
The present invention relates to a method of driving liquid crystals and a display apparatus therefor, and in particular to a driving method of displaying STN (Super Twisted Nematic) liquid crystals with high contrast and a display apparatus therefor.
2. Description of the Related Art
As a conventional driving method of liquid crystal display apparatus having a matrix structure, there is known a technique described in "Ultimate Limits for Matrix Addressing of RMS-Responding Liquid-Crystal Displays," IEEE Transactions on Electron Devices, Vol. ED-26, No. 5, May 1979 (pp. 795-802) and "Active Addressing Method for High-Contrast Video-Rate STN Displays," SID 92 DIGEST, pp. 228-231. According to this technique, each row electrode is provided with voltage depending upon an orthogonal function, whereas each column electrode is provided with voltage depending upon a function obtained as sum of products of every display information of that column and a function of the scanning side. The driving method will hereafter be described in detail by referring to FIGS. 1 to 4.
FIG. 1 shows the structure of a liquid crystal display panel having a matrix structure consisting of N rows by M columns. An intersection of a row electrode and a column electrode forms a dot D(i,j). A voltage represented by a function f(i) (i=1, 2, . . . N) is supplied to each of N row electrodes. A voltage represented by a function g(i) (i=1, 2, . . . M) is supplied to each of M column electrodes. U(i,j) denotes voltage supplied to the dot D(i,j). The voltage U(i,j) is a difference between values of the voltage functions f(i) and g(i). In the ensuing description, voltage is normalized. FIG. 2 is a diagram showing an example of orthogonal function voltage supplied to row electrodes to drive STN liquid crystal displays. This example is generally used at the present time. Assuming now that the function f(i) is represented by FIG. 2, the functions f(i) and g(i) can be represented by equations (1) and (2), respectively. EQU f(i)=FP.multidot..delta.(i,t) (1) ##EQU1## In the equations (1) and (2), .delta.(i,t) is 1 for i=t and 0 for i.noteq.j. FP is a constant given by the following equation (3). ##EQU2##
P(i,j) denotes display information of the dot D(i,j). P(i,j) is -1 for display-on state and 1 for display-off state. By using equations (1), (2) and (3), the effective voltage U.sub.rms (i,j) applied to the dot D(i,j) at this time can be represented by the following equation (4). ##EQU3## Letting T=N and rewriting (4) gives ##EQU4## From equations (5), (6) and (7), therefore, the effective voltage U.sub.rms (i,j) can be written as ##EQU5## Assuming that the dot D(i,j) is in the display-on state, P(i,j)=-1 and the effective voltage U.sub.rms (i,j) is represented by equation (9). Assuming that the dot D(i,j) is in the display-off state, P(i,j)=1 and the effective voltage U.sub.rms (i,j) is represented by equation (10). ##EQU6## Voltage applied to the dot D(i,j) is (f(i)-g(j)) and has a waveform as shown in FIG. 3 on the basis of equations (1) and (2). In FIG. 3, S1, S2 and S3 are represented by the following equations. ##EQU7## Assuming now that N=240, we get S1=12.1 (when D(i,j)=display on), S1=10.6 (when D(i,j)=display off), S2=0.73, and S3=-0.73. As a result, a large voltage is applied once (i=t) during one frame (i.e., a period of t=1 to N) and low voltage is applied during the remaining intervals. In fast responding STN liquid crystal displays, the display luminance lowers while this low voltage is being applied.
As a driving method avoiding this, a method described below has been proposed. FIG. 4 shows an orthogonal function called Walsh function. In the example shown in FIG. 4, the number of divisions is 8. Assuming now that a Walsh function with the number of divisions being equivalent to T is used as the function f(i) of voltage applied to row electrodes of the liquid crystal display panel of FIG. 1 and N Walsh functions are selected out of T Walsh functions (T.gtoreq.N) and used as the function f(i), the effective voltage value U.sub.rms (i,j) of the dot (i,j) is derived.
It is assumed that the functions f(i) and g(j) are represented by the following equations (15) and (16). EQU f(i)=FP.multidot.W(i,t) (15) ##EQU8## In these equations, W(i,t) is a Walsh function and has a value of 1 or -1. FP is a constant indicated by equation (17). ##EQU9##
In equation (4), ##EQU10##
The effective voltage U.sub.rms (i,j) of the dot D(i,j) becomes ##EQU11##
As evident from the results heretofore described, the effective voltage U.sub.rms (i,j) obtained when the Walsh function is used becomes identical with equation (8). U.sub.rms (i,j) has a value of equation (9) for display-on state, whereas U.sub.rms (i,j) has a value of equation (10) for display-off state.
In this case, g(j) of equation (16) is rewritten as ##EQU12## where D is the number of coincident values of P(i,j) with respect to W(i,j) with i=1 to N in the j-th column (P(i,j) assumes a value of .+-.1, and W(i,j) assumes a value of .+-.1). At this time, the value of D has normal distribution represented by the following equation. ##EQU13##
As indicated by equation (23), D follows normal distribution around N/2. Therefore, equation (22) also follows normal distribution in the same way. As compared with FIG. 3, therefore, average voltage over the period of t=1 to T is applied to the dot D(i,j) as the voltage waveform (f(i)-g(j)).
D can assume a value ranging from 0 (complete noncoincidence) to N (entire coincidence). From equation (22), the peak value of g(j) becomes ##EQU14## Furthermore, g(j) can assume a value out of N+1 levels. Regarding this liquid crystal display device as a display device for a personal computer, N=240 rows are needed. As the column voltage g(j), therefore, a liquid crystal driver generating 241 levels and generating a peak voltage of approximately 22.65 volts (in case the nonselection voltage of the liquid crystal display is 1 volt) on the basis of equation (23) is needed. Since it is difficult to realize such a liquid crystal driver, it is said that the liquid crystal driver having 64 levels (where the peak voltage is 5.95 volts) is sufficient on the basis of the property of D following the normal distribution. In this case, however, overflow, i.e., voltage exceeding 64 levels might be needed with a probability of once every 115 frames. However, it is said that overflow occurs very rarely in actual display and hence there is no problem in the above described conventional technique.
If a Walsh function is used as the voltage function supplied to the row electrodes in the above described driving method, however, the voltage function g(j) supplied to the column electrodes becomes as represented by the following equation (24) on the basis of equations (15) and (16). For determining the voltage applied to one dot at certain time t, it is necessary to calculate the sum of products of display information P(i,j) with i=1 to N and the Walsh function (i,t). Its realization is difficult, and a concrete driving circuit is not shown clearly. ##EQU15## Assuming that the voltage function supplied to the row electrodes is the function shown in FIG. 2, the voltage function g(j) applied to the column electrodes is represented by the following equation (25). ##EQU16## The product summation thus becomes unnecessary and the circuit configuration becomes simple. In this case, however, the voltage waveform applied to the dot D(i,j) assumes high voltage during only one interval in N intervals, and assumes low voltage during remaining N-1 intervals. In case of fast responding STN liquid crystal displays, therefore, the contrast drops.
Furthermore, in the conventional technique, a liquid crystal driver generating the column voltage is requested to have N+1 levels and the peak voltage expressed by equation (23). However, it is said that a liquid crystal driver having 64 levels and approximately 5.95 volts suffices for the personal computer display of N=240, considering the property of the value assumed by D. Therefore, overflow occurs with a probability of once every 115 frames. In this case, it is considered that overflow occurs with a probability defined by the normal distribution following the above described theory when the contents of display change momentarily as in moving picture display. In displays used for information processing devices such as personal computers or work statios, however, contents of displays are not moving pictures but still pictures in many cases. If overflow occurs once in a still picture, therefore, overflow occurs in every frame and D loses its property following the normal distribution. Therefore, the effective value of the pertinent column electrode voltage decreases and the quality of display is degraded.